1. Field of the Invention
The present invention relates to a shock sensor and more particularly to a shock sensor for use in an air bag system for automobiles.
2. Description of Prior Art
An air bag system is employed in an automobile in order to protect the driver from a shock when the automobile collides with another automobile or the like. A shock sensor detects a shock and outputs a detection signal. The detection signal activates a firing device of an air bag which in turn causes the air bag to quickly inflate to protect the driver from the shock.
One such type of prior art shock sensor is disclosed in Japanese Patent Preliminary Publications No. 7-198738 and No. 8-29444.
FIG. 12 illustrates the general construction of a prior art reed switch type shock sensor, shown in JPA No. 8-29444, which is used in an air bag system for automobiles.
Referring to FIG. 12, a magnet 14 is housed in a case 11 and held in position by a spring 13. A reed switch 12 extends through the spring 13 and is loosely received in the magnet 14.
The reed switch 12 in the case 11 is open since the magnet 14 is located outside of an area in which the reed switch 12 is responsive to the magnetic field of the magnet 14.
A shock applied to the shock sensor, for example, in a direction shown by an arrow A imposes an acceleration on the mass of the spring 13. If the acceleration is large enough to overcome the resilient force of the spring 13, the magnet 14 moves into the area in which the reed switch 12 is responsive to the magnetic field of the magnet 14 to close its contacts.
However, a problem with the prior art reed switch type shock sensor is that the sensor is sensitive to shocks only in the direction shown by arrow A in FIG. 12. Another problem is that the sensitivity of the sensor varies with the weight, dimension, and magnetic flux density of the magnet 14. Further, the use of a reed switch as a shock-detecting element is an obstacle to miniaturizing the shock sensor.